concrete techology 4

8/11/2019 Concrete Techology 4

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4C H A P T E R

W

ater is an important ingredient of concrete as it

actively participates in the chemical reactionwith cement. Since it helps to form the strength giving

cement gel, the quantity and quality of water is

required to be looked into very carefully. It has been

discussed enough in chapter 1 about the quantity of

mixing water but so far the quality of water has not

been discussed. In practice, very often great control

on properties of cement and aggregate is exercised,

but the control on the quality of water is often

neglected. Since quality of water affects the strength,

it is necessary for us to go into the purity and quality

of water.

Qualities of Water

A popular yard-stick to the suitability of water for

mixing concrete is that, if water is fit for drinking it is

fit for making concrete. This does not appear to be a

true statement for all conditions. Some waters

containing a small amount of sugar would be suitable

for drinking but not for mixing concrete and

conversely water suitable for making concrete may

not necessarily be fit for drinking. Some specifications

Water

!!!!! Qualities of Water

!!!!! Use of Sea Water for Mixing

Concrete

Water requires to be tested to find out its suitability for large

projects.

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require that if the water is not obtained from source that has proved satisfactory, the strength

of concrete or mortar made with questionable water should be compared with similarconcrete or mortar made with pure water. Some specification also accept water for making

concrete if the pH value of water lies between 6 and 8 and the water is free from organic

matter. Instead of depending upon pH value and other chemical composition, the best course

to find out whether a particular source of water is suitable for concrete making or not, is to

make concrete with this water and compare its 7 days and 28 days strength with companioncubes made with distilled water. If the compressive strength is upto 90 per cent, the sourceof water may be accepted. This criteria may be safely adopted in places like coastal area ofmarshy area or in other places where the available water is brackish in nature and of doubtfulquality. However, it is logical to know what harm the impurities in water do to the concreteand what degree of impurity is permissible is mixing concrete and curing concrete.

Carbonates and bi-carbonates of sodium and potassium effect the setting time of cement.While sodium carbonate may cause quick setting, the bi-carbonates may either accelerate orretard the setting. The other higher concentrations of these salts will materially reduce theconcrete strength. If some of these salts exceeds 1,000 ppm, tests for setting time and 28 daysstrength should be carried out. In lower concentrations they may be accepted.

Brackish water contains chlorides and sulphates. When chloride does not exceed 10,000ppm and sulphate does not exceed 3,000 ppm the water is harmless, but water with even

higher salt content has been used satisfactorily.

Salts of Manganese, Tin, Zinc, Copper and Lead cause a marked reduction in strength

of concrete. Sodium iodate, sodium phosphate, and sodium borate reduce the initial strength

of concrete to an extra-ordinarily high degree. Another salt that is detrimental to concrete is

sodium sulphide and even a sulphide content of 100 ppm warrants testing.

Silts and suspended particles are undesirable as they interfere with setting, hardening and

bond characteristics. A turbidity limit of 2,000 ppm has been suggested. Table 4.1 shows the

tolerable concentration of some impurities in mixing water.

Underground water is sometime found unsuitable for mixing or even for

curing concrete.

The quality of underground water is to be checked.


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The initial setting time of the test block made with a cement and the water proposed

to be used shall not differ by 30 minutes from the initial setting time of the test block madewith same cement and distilled water.

Table 4.1.Tolerable Concentrations of Some Impurities in Mixing Water

ImpurityImpurityImpurityImpurityImpurity Tolerable ConcentrationTolerable ConcentrationTolerable ConcentrationTolerable ConcentrationTolerable Concentration

Sodium and potassium : 1,000 ppm (total). If this is exceeded, it is

carbonates and bi-carbonates advisable to make tests both for setting time and

28 days strength

Chlorides : 10,000 ppm.

Sulphuric anhydride : 3,000 ppm

Calcium chloride : 2 per cent by weight of cement in non-pre-

stressed concrete

Sodium iodate, sodium

sulphate, sodium : very low

arsenate, sodium borate

Sodium sulphide : Even 100 ppm warrants testing

Sodium hydroxide : 0.5 per cent by weight of cement, provided quick

set is not induced.

Salt and suspended particles : 2,000 ppm. Mixing water with a high content ofsuspended solids should be allowed to stand in a s

ettling basin before use.

Total dissolved salts : 15,000 ppm.

Organic material : 3,000 ppm. Water containing humic acid or such

organic acids may adversely affect the hardening

of concrete; 780 ppm. of humic acid are reported to

have seriously impaired the strength of concrete.

In the case of such waters therefore, further testing

is necessary.

pH : shall not be less than 6

The following guidelines should also be taken into consideration regarding the quality

of water.

(a) To neutralize 100 ml sample of water using phenoplhaline as an indicator, it should

not require more than 5 ml of 0.02 normal NaOH.

(b) To neutralise 100 ml of sample of water, using mixed indicator, it should not require

more than 25 ml of 0.02 normal H2So4.

(c) Permissible limits for solids is as given below in table 4.2.


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Table 4.2. Permissible limit for solids as per IS 456 of 2000

MaterialMaterialMaterialMaterialMaterial Tested as perTested as perTested as perTested as perTested as per Permissible limit Max.Permissible limit Max.Permissible limit Max.Permissible limit Max.Permissible limit Max.

Organic IS 3025 (pt 18) 200 mg/l

Inorganic IS 3025 (pt 18) 3000 mg/l

Sulphates IS 3025 (pt 24) 400 mg/l

(as So3)

Chlorides IS 3025 (pt 32) 2000 mg/l for concrete work not con-

(as Cl) taining embedded steel and 500 mg/l

for reinforced concrete work

Suspended IS 3025 (pt 17) 2000 mg/l

Algae in mixing water may cause a marked reduction in strength of concrete either by

combining with cement to reduce the bond or by causing large amount of air entrainment

in concrete. Algae which are present on the surface of the aggregate have the same effect

as in that of mixing water.

Use of Sea Water for Mixing Concrete

Sea water has a salinity of about 3.5 per cent. In that about 78% is sodium chloride and

15% is chloride and sulphate of magnesium. Sea water also contain small quantities of sodium

and potassium salts. This can react with reactive aggregates in the same manner as alkalies

in cement. Therefore sea water should not be used even for PCC if aggregates are known to

be potentially alkali reactive. It is reported that the use of sea water for mixing concrete doesnot appreciably reduce the strength of concrete although it may lead to corrosion of

reinforcement in certain cases. Research workers are unanimous in their opinion, that sea

water can be used in un-reinforced concrete or mass concrete. Sea water slightly accelerates

the early strength of concrete. But it reduces the 28 days strength of concrete by about 10

to 15 per cent. However, this loss of strength could be made up by redesigning the mix. Water

containing large quantities of chlorides in sea water may cause efflorescence and persistent

dampness. When the appearance of concrete is important sea water may be avoided. The use

of sea water is also not advisable for plastering purpose which is subsequently going to be

painted.

Divergent opinion exists on the question of corrosion of reinforcement due to the use of

sea water. Some research workers cautioned about the risk of corrosion of reinforcement

particularly in tropical climatic regions, whereas some research workers did not find the riskof corrosion due to the use of sea water. Experiments have shown that corrosion of

reinforcement occurred when concrete was made with pure water and immersed in pure

water when the concrete was comparatively porous, whereas, no corrosion of reinforcement

was found when sea water was used for mixing and the specimen was immersed in salt water

when the concrete was dense and enough cover to the reinforcement was given. From this

it could be inferred that the factor for corrosion is not the use of sea water or the quality of

water where the concrete is placed. The factors effecting corrosion is permeability of concrete

and lack of cover. However, since these factors cannot be adequately taken care of always at

the site of work, it may be wise that sea water be avoided for making reinforced concrete. For


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economical or other passing reasons, if sea water cannot be avoided for making reinforced

concrete, particular precautions should be taken to make the concrete dense by using low

water/cement ratio coupled with vibration and to give an adequate cover of at least 7.5 cm.

The use of sea water must be avoided in prestressed concrete work because of stress corrosion

and undue loss of cross section of small diameter wires. The latest Indian standard IS 456 of2000 prohibits the use of Sea Water for mixing and curing of reinforced concrete and

prestressed concrete work. This specification permits the use of Sea Water for mixing and

curing of plain cement concrete (PCC) under unavoidable situation..

It is pertinent at this point to consider the suitability of water for curing. Water that

contains impurities which caused staining, is objectionable for curing concrete members

whose look is important. The most common cause of staining is usually high concentration

of iron or organic matter in the water. Water that contains more than 0.08 ppm. of iron may

be avoided for curing if the appearance of concrete is important. Similarly the use of sea water

may also be avoided in such cases. In other cases, the water, normally fit for mixing can also

be used for curing.

Sea water is not to be used for prestressed concrete or for reinforced concrete.

If unavoidable, it could be used for plain cement concrete (PCC).

concrete techology 4

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